In recent years, a decrease in size and weight, an increase in power output and an increase in efficiency have been required in a permanent magnet motor used in a hybrid car, a hard disk drive, or the like. To realize such a decrease in size and weight, an increase in power output and an increase in efficiency in the permanent magnet motor mentioned above, film-thinning and a further improvement in magnetic performance are required of a permanent magnet to be buried in the permanent magnet motor. Meanwhile, as permanent magnet, there have been known ferrite magnets, Sm—Co-based magnets, Nd—Fe—B-based magnets, Sm2Fe17Nx-based magnets or the like. As permanent magnet for permanent magnet motor, there are typically used Nd—Fe—B-based magnets due to remarkably high residual magnetic flux density.
As method for manufacturing a permanent magnet, a powder sintering process is generally used. In this powder sintering process, raw material is coarsely milled first and furthermore, is finely milled into magnet powder by a jet mill (dry-milling) method. Thereafter, the magnet powder is put in a mold and pressed to form in a desired shape with magnetic field applied from outside. Then, the magnet powder formed and solidified in the desired shape is sintered at a predetermined temperature (for instance, at a temperature between 800 and 1150 degrees Celsius for the case of Nd—Fe—B-based magnet) for completion.
On the other hand, as to Nd-based magnets such as Nd—Fe—B magnets, poor heat resistance is pointed to as defect. Therefore, in case a Nd-based magnet is employed in a permanent magnet motor, continuous driving of the motor brings the magnet into gradual decline of coercive force and residual magnetic flux density. Then, in case of employing a Nd-based magnet in a permanent magnet motor, in order to improve heat resistance of the Nd-based magnet, Dy (dysprosium) or Tb (terbium) having high magnetic anisotropy is added to further improve coercive force.
Meanwhile, the coercive force of a magnet can be improved without using Dy or Tb. For example, it has been known that the magnetic performance of a permanent magnet can be basically improved by making the crystal grain size in a sintered body very fine, because the magnetic characteristics of a magnet can be approximated by a theory of single-domain particles. Here, in order to make the grain size in the sintered body very fine, a particle size of the magnet raw material before sintering also needs to be made very fine. However, even if the magnet raw material finely milled into a very fine particle size is compacted and sintered, grain growth occurs in the magnet particles at the time of sintering. Therefore, after sintering, the crystal grain size in the sintered body increases to be larger than the size before sintering, and as a result, it has been impossible to achieve a very fine crystal grain size. In addition, if the crystal grain has a larger size, the domain walls created in a grain easily move, resulting in drastic decrease of the coercive force.
Therefore, as a means for inhibiting the grain growth of magnet particles, there is considered a method of adding a substance for inhibiting the grain growth of the magnet particles (hereinafter referred to as a grain growth inhibitor), to the magnet raw material before sintering. According to this method, for example, the surface of a magnet particle before sintering is coated with the grain growth inhibitor such as a metal compound whose melting point is higher than the sintering temperature, which makes it possible to inhibit the grain growth of magnet particles at sintering. In JP Laid-open Patent Application Publication No. 2004-250781, for example, phosphorus is added as grain growth inhibitor to the magnet powder.